The invention relates to optical fibers, and more specifically to long term stability in the transmission properties at 1380-1400 nm of optical fibers used in telecommunication cables.
Optical telecommunication is usually conducted with infrared light in the wavelength ranges of 0.8-0.9 xcexcm or 1.3-1.6 xcexcm. These wavelengths are sufficiently generated by LEDs, laser diodes and suffer least attenuation in the fibers.
A problem associated with operating fibers in this transmission window is the fact that absorption bands occur in this wavelength region. These absorption bands are in particular due to the presence of OH groups.
It was suggested to use highly pure silica with a low hydroxyl content to produce the optical fiber. Optical fibers contain nowadays typically less than 0.1 ppm OH.
However, it has been observed that even fibers of very low hydroxyl silica, when exposed to hydrogen at ambient temperature show an increase in attenuation in the transmission window of 1.3-1.6 xcexcm, and in particular at 1380-1400 nm. This increase of attenuation with time due to the presence of hydrogen is often referred to xe2x80x9chydrogen aging lossxe2x80x9d.
While diffusion of molecular hydrogen into the fiber also creates absorption bands, the corresponding loss is not permanent and may be removed by degassing.
However, diffusion of molecular hydrogen also leads to irreversible reactions. This type of loss is referred to as permanent.
The hydrogen diffusion is observed even through the fiber cladding, and once the fibers are bundled into telecommunication cables. It is already observed upon exposure during some days to 0.01 atmospheres of hydrogen at ambient temperature. The increment of attenuation due to permanent hydrogen aging loss may be evaluated to 0.02 to 0.12 dB/km at 1383 nm. Exposure to such traces of hydrogen is difficult to avoid. One source of hydrogen may be corrosion phenomena due to the presence of dissimilar metals and moisture in the cable. Also, hydrogen is believed to be produced by some types of silicone upon heating. Optical fibers exposed to seawater and air in particular undergo a large increase of the attenuation with time. The permanent hydrogen aging loss is therefore highly undesirable because it strongly affects the fiber transmission properties Ensuring low attenuation and temporal attenuation stability across the spectral range requires therefore mastering the permanent effects of hydrogen diffusion into the optical fiber.
In order to reduce the loss due to OH absorption bands, it has been proposed in different publications to treat the optical fibers with deuterium in order to replace the hydrogen of OH groups by deuterium either at high temperature or by irradiation.
Isotope exchange between hydrogen and deuterium is reported either at high temperature (above 400xc2x0 C.) or by irradiation [B. Kumar, xe2x80x9cIsotope exchange reactions in vitreous silicaxe2x80x9d, Physics and chemistry of glasses Vol. 26 No6 (1985), 213-216]. During this reaction, the hydrogen of an OH group is replaced by a deuterium. The organic polymers used for the coating of optical fibers do however not in general withstand temperatures necessary for such a reaction. The high temperature process is therefore not usable for reducing the hydrogen aging loss in optical fibers.
U.S. Pat. No. 4,515,612 to Burrus describes a method wherein a thermally induced hydrogen/deuterium exchange is carried out on the optical fiber preform. However, this approach does not prevent attenuation loss due to hydrogen diffusion at a later stage and does thus not ensure reliability during service life time.
U.S. Pat. No. 4,685,945 to Freund describes a method of reacting peroxide linkages existing in the fiber with molecular deuterium (D2) at temperature compatible with the fiber. It is proposed to let the fiber be permeated by the deuterium at a temperature close to the temperature where increased loss or degradation occurs. Simultaneously or subsequently, the reaction with deuterium is stimulated through a light activation step with intense light. The low hydroxyl silica available at that time had a notably larger OH content.
Recently, a new hydrogen aging mechanism has been evidenced [xe2x80x9cNew hydrogen aging loss mechanism in the 1400 nm windowxe2x80x9d, K. H. Chang, D. Kalish and M. L. Pearsall, Proceedings OFC 1999]. This mechanism involves very reactive defects, a limited number of which exist in an optical fiber. Some of these defects are believed to correspond to peroxide defects, i.e. a deviation in the glass structure due to the insertion of an oxygen atom between a Sixe2x80x94Oxe2x80x94Si bond but other defects may be involved in the mechanism These defects may react with molecular hydrogen to yield additional OH groups not present before in the material. Such a mechanism explains the observation of hydroxyl groups in a virtually hydroxyl free silica upon short exposure to hydrogen. Such a process creates new OH groups in the material and has detrimental effects on the attenuation, notably because it gives rise to an increase of the SiOH peak, situated at 1383 nm. The reaction is irreversible ; while further exposure to hydrogen does not lead to further reaction, heating the fiber may not reverse the reaction. The reaction is further fast. A step like increase in attenuation is noted when a fiber is exposed to partial pressures of hydrogen such as 1% for time periods of a few days in ambient conditions. The magnitude and time of onset are highly dependent on the nature of the fiber chosen. The article of K. H. Chang, D. Kalish and M. L. Pearsall discloses this aging mechanism but does not provide any practical suggestion as how to reduce hydrogen aging loss of optical fibers.
The invention addresses the problem of hydrogen aging loss of optical fibers.
According to a preferred embodiment, the gas mixture comprises 0.01 to 100%, preferably 0.5 to 2% of deuterium.
Preferably, the gas mixture further comprises nitrogen.
According to a preferred embodiment, the optical fiber is contacted with the gas mixture during a time period of 1 day to 2 weeks, preferably 3 to 10 days. The temperature of the reaction is preferably from 20 to 40xc2x0 C.
It is particularly advantageous to carry out the degassing of the reacted optical fiber by maintaining it in air or nitrogen. Preferably, the reacted optical fiber is degassed during a time period of 1 to 3 weeks.
It has been found advantageous to carry out the reaction in a sealable vessel. It is particularly useful if the gas mixture is subsequently recovered from the reaction vessel.
The invention also provides an optical fiber treated by the method according to the invention.
Finally, the invention provides a telecommunication cable comprising such an optical fiber with low hydrogen aging loss.
The invention thus provides a simple method to reduce the hydrogen sensitivity of optical fibers at low cost. It also provides an optical fiber with reduced hydrogen aging loss and enhanced attenuation stability throughout the service life time. It finally provides telecommunication cables containing these fibers which are highly reliable even in difficult environmental conditions.